Communication receiver incorporating tone operated
pulser circuit and electronic switch

ABSTRACT

1. A COMMUNCIATION RECEIVER FOR RECEIVING CARRIER SIGNALS MODULATED BY AT LEAST ONE CONTROL TONE, SAID RECEIVER COMPRISING A PROCESSING CIRCUIT FOR RECEIVING THE MODULATED CARRIER SIGNALS AND DETECTING THE CONTROL TONE THEREIN, A PULSER CIRCUIT COUPLED TO SAID PROCESSING CIRCUIT AND OPERATIVE TO PRODUCE A SERIES OF PULSES FOR INTERMITTENTLY RENDERING SAID PROCESSING CIRCUIT OPERATIVE, A DECODER CIRCUIT COUPLED TO SAID PROCESSING CIRCUIT AND HAVING FIRST AND SECOND OUTPUTS, SAID DECODER CIRCUIT PRODUCING AT SAID FIRST OUTPUT A FIRST CONTROL SIGNAL COMMENCING WITH THE INITIATION OF THE CONTROL TONE AND PRODUCING AT SAID SECOND OUTPUT A SECOND CONTROL SIGNAL COMMENCING A PREDETERMINED TIME AFTER INITIATION OF THE CONTROL TONE, SAID PULSER CIRCUIT HAVING AN INPUT COUPLED TO THE FIRST OUTPUT OF SAID DECODER CIRCUIT AND RESPONSIVE TO THE APPLICATION THERETO OF SAID FIRST CONTROL SIGNAL TO FURNISH A CONTINUOUS SUPPLY VOLTAGE FOR SAID PROCESSING CIRCUIT FOR AN INTERVAL SUBSTANTIALLY LONGER THAN THE DURATION OF EACH PULSE IN SAID SERIES OF PULSES, AN ELECTRONIC SWITCH CIRCUIT COUPLED TO THE SECOND OUTPUT OF SAID DECODER CIRCUIT AND RESPONSIVE TO SAID SECOND CONTROL SIGNAL TO PROVIDE AN ENABLING SIGNAL, A UTILIZATION CIRCUIT HAVING AN INPUT COUPLED TO SAID SWITCH CIRCUIT AND RESPONSIVE TO THE ENABLING SIGNAL TO PROVIDE AN OUTPUT SIGNAL, AND AN ANNUNCIATOR COUPLED TO SAID UTILIZATION CIRCUIT FOR CONVERTING THE OUTPUT SIGNAL INTO USABLE FORM.

Re. 28, 222 ING TONE OPERATED CTRONIC SWITCH NOV- 5. 1974 K. H. wYcor-FCOIHUNICATION RECEIVER INCORPORAT PULSE!! CIRCUIT AND ELE Original FiledSept. 29. 1969 9 Sheets-Sheet 1 lnvenor KEITH H. WYCOFF /0 .uma M Etz:.1.5.2 EXE l N .omo

com n wm fom F os m24 o sw mm2; A n QN o@ ow o@ Tail). o mEI Vommm om. HO m ki omJ wm. mmwd 508m@ mm K. H. wYcor-'F Re. 28, 222 COIHUNICATIONRECEIVER INCOREORATING TONE OPERATED Nov. 5, 1974 PuLsEn CIRCUIT .umELECTRONIC SWITCH original Filed swt. 29, 1969 9 Sheets-Sheet 2 +m +mJIFIII llllllllllll. rTow.

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NOV. 5, K, H, WYCOFF Re. 28, 222

COMMUNICATION RECEIVER INCORPORAIING TONE OPERATED PULSER CIRCUIT ANDELECTRONIC SWITCH FIG. 4

NOV. 5, K- H, WYCOFF Re. 28, 222

COMMUNICATION RECEIVER INCORPORATING TONE OPERATLD PULSER CIRCUIT ANDELECTRONIC SWITCH Original Filed Sept. 29, 1969 9 Sheets-Sheet 4 Nov. 5,1974 Original Filed Sept. 29, 1969 9 Sheets-Sheet 5 ZBL qu 3953L 403 39o394 4o V-B+ 4uj ,1 34W@ |392 l l 396 4|3 l L@ l E+ J 393 438\ 4m,d 2X414 j -2 5-246 29o 294 30o -'E285 302 3u 2 247 29| :All w I 253 29 I 3|3I 1li a: 293 i 25o 22 303 31o FIG. 7

Nov. 5, 1974 K. H, WYCOFF Re. 28, 222

COKIUHICATION RECEIVER INCORPORATING TONE OPERATED PULSER CIRCUIT ANDELECTRONIC SWITCH Original Filed Sept. 29, 1969 9 Sheets-Sheet FIG. 8

Re. 28, 222 ERATED K. H. WYCOFF Nov. 5, 1974 COMMUNICATION RECEIVERINCORFORATING TONE OP PULSER CIRCUIT AND ELECTRONIC SWITCH OriginalFiled Sept. 29, 1969 9 Sheets-Sheet 7 Nov. 5, 1974 Original Filed Sept.29, 1969 COKIUNICAT ION RECEIVER lNCURkORA'LING TONE OPEHATBD PULSERCIRCUIT AND ELECTRUNIC SWITCH 9 Sheets-Sheet f CQ N i 1T N n n rf) C 8 Dm i, ""f N 1 l0 J r--f f-f AJ FJ J x n', (1 /l l0 N Q i r f O N LD N N(D FIG. IO

Nov. 5, 1974 K WYCQFF Re'. 28, 222

\ CCIIUNICATIOR RECEIVER INCCRFCRATING TONE CPERATBD PULsER CIRCUIT ANDELECTRONIC SWITCH Original Filed Sept. 29, 1969 9 Sheets-Sheet 9 UnitedStates Patent O Int. Cl. H0411 I 06 U.S. Cl. 325-492 26 Claims Matterenclosed in heavyr brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE A communication receiver comprising aprocessing circuit for receiving modulated carrier signals and detectingone or more control tones therein, a pulser circuit coupled to theprocessing circuit and operative to produce a series of pulses forintermittently rendering the processing circuit operative, and a decodercircuit coupled to the processing circuit and responsive to the controltone or tones for generating at the output thereof a control signal, thepulser circuit being coupled to the output of the decoder circuit andresponsive to the application thereto of the control signal to furnish acontinuous supply voltage for the processing circuit for an intervalsubstantially longer than the duration of each pulse in the series ofpulses.

The present invention is directed to communication receivers, andparticularly to a communication receiver incorporating therein a pulsercircuit to maximize battery life and to provide other useful functionsin the receiver.

It is an important object of the present invention to provide acommunication receiver for receiving signals modulated by at least onecontrol tone, the receiver comprising a processing circuit for receivingmodulated carrier signals and detecting one or more control tonestherein, a pulser circuit coupled to the processing circuit andoperative to produce a series of pulses for intermittently renden ingthe processing circuit operative, and a decoder circuit coupled to theprocessing circuit and responsive to the control signal, the pulsercircuit being coupled to the output of the decoder circuit andresponsive to the application thereto of the control signal to furnish acontinuous supply voltage for the processing circuit for an intervalsubstantially longer than the duration of each pulse in the series ofpulses.

In connection with the foregoing object, it is another object of theinvention to provide a receiver comprising a switch circuit coupled tothe tone decoder circuit and responsive to the control signal developedby the decoder circuit to provide an enabling signal, a utilizationcircuit coupled to the switch circuit and responsive to the presence ofthe enabling signal to provide an output signal, and an annunciatorcoupled to the utilization circuit for converting the output signal intousable form.

In connection with the foregoing object, it is another object of theinvention to provide a communication receiver in which the utilizationcircuit consists of an oscillator circuit and/ or an audio amplifier andthe annunciator consists of a lamp and/ or a loudspeaker.

Still another object of the present invention is to provide a pulsatingsupply voltage for intermittently operating an RF signal processingcircuit, and a pulse extender to increase the length of the pulses foroperation of an annunciator system.

Yet another object of the present invention is to provide acommunication receiver including a pulser circuit that generates aseries of pulses of one frequency in response Re. 28,222 Reissued Nov.5, 1974 to a set of control tones and a different frequency in responseto a set of control tones having a different makeup.

A further object of the present invention is to provide a communicationreceiver having a pulser circuit that generates a pulsating signal forintermittently operating an alerting device such as a lamp or aloudspeaker.

A still further object of the present invention is to provide acommunication receiver having a pulser circuit for producing a pulsatingsignal during standby and a continuous signal when audio information isbeing received, the audio amplifier being inoperative during standby andbeing rendered operative by the continuous supply voltage foramplification of the audio information.

Another object of the present invention is to maximize the useful lifeof a battery used in a communication receiver, while maintaining smallsize and light weight.

Still another object of the present invention is to provide abattery-saving circuit that is highly sensitive, highly ei'licient inrespect to minimizing current drain, and permits high speed signaling toaccommodate maximum use of the available spectrum.

Yet another object of the present invention is to provide abattery-saving circuit which wil] intermittently energize the receiverduring standby but will continuously energize the receiver when a propercontrol tone or series of control tones is received, and will beaffected only negligibly by control tones other than those to which thereceiver is to respond.

A further object of the invention is to provide a paging system whichwill not only provide an audible alerting tone for the user of thepager, but will also provide a visual alerting signal to enable use ofthe system in noisy locations.

In connection with the foregoing object, it is a still further object tominimize drain on the battery when the visual and audible alertingsignals are generated.

Further features of the invention pertain to the particular arrangementof the elements of the communication receiver, and the components andelements thereof, whereby the above-outlined and additional operatingfeatures thereof are obtained.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof will best beunderstood by reference to the following specification taken inconnection with the accompanying drawings, in which:

FIG. 1 is a block diagram of a communication receiver made in accordancewith and embodying the principles of the present invention;

FIG. 2 is a more detailed block diagram with respect to the receivercircuits and depicts schematically the decoder for responding to asingle control tone;

FIG. 3 is a schematic diagram of the pulser circuit, the feedbacknetwork, the timer switch and the audio amplitier in the drawing of FIG.l;

FIG. 4 is a graph showing the signals at various points in the circuitryof FIG. 3;

FIG. 5 is a block diagram of a second embodiment of the presentinvention;

FIG. 6 is a detailed block diagram of the decoder circuit of FIG. 5;

FIGS. 7, 8 and 9 are detailed schematics of the certain portions of thecommunication receiver of FIG. 5;

FIG. l0 is a graph depicting the signals at various points in thecircuitry shown in FIG. 9; and

FIG. 11 shows an additional system that may be used in conjunction withthe receiver of FIG. 9. 1

The principles of the present invention are equally applicable tocommunication systems utilizing lines, modulated supersonic signals, AMradio signals, and FM radio signals. For illustrative purposes, there isshown in the drawings, a communication system employing FM radiosignals. Those skilled in the art will readily understand that thevarious principles to be described hereinafter in conjunction with thesystem employing FM radio signals can be readily adjusted to the othertypes of communications systems using other forms of transmissions suchas those set forth above.

Referring now to FIG. l of the drawings, there is shown a. communicationreceiver 20 made in accordance with and embodying the principles of thepresent inven tion, the receiver being generallyr designated by thenumeral 20. The receiver 20 is adapted to receive an RF carriermodulated by audio signals and one or more control tones. Thetransmissions are received by the receiver 20 at the antenna 2l thereofand are coupled to receiver circuits 30 which process the modulated RFcarrier and converted it into a demodulated composite signal com* prisedof the audio signals and the control tone. The output from the receivercircuits 3i) appears on a conductor 42 which applies the audio signalsto the audio amplifier d 160, which in turn has its output coupled viaconductor 177 to a speaker 180.

In accordance with the present invention, a decoder Stl is coupled tothe conductor 42 and generates a control signal on the conductor 96 whenthe modulated RF cari rier received by the receiver 20 includes thecontrol tone to which the decoder 50 responds. Connected to the decoder50 via a conductor 96 is n pulser circuit 120, the pulser circuit 120producing on the conductor 136 a series of pulses for intermittentlyrendering the receiver circuits 30 (and the decoder 50, if desired)operative. ln an operative embodiment. the pulses had a duration l5milliseconds and the interval between pulses was 300 milliseconds, thereceiver circuits 30 being rendered operative during the millisecondduration of a pulse. If a modulated RF j carrier is impressed on theantenna 2l while the receiver circuits are operative, that is, duringthe presence of a pulse on the conductor 136, the receiver circuits 30will become operative for a given time interval to process and detectthe tones and the information and translate it to the decoder 50. 1f thesignal contains the control lone to which the decoder 5t) is tuned, acontrol signal will appear on the conductor 96, which energizes thepulser circuit 120 to cause it to provide a continuous supply voltagefor a predetermined time. The supply voltage on the conductor r 136renders the receiver circuits 30 operative for that predetermined time.As indicated by the dashed line, the series of pulses (or continuoussupply voltage, depending on which is present) may be applied to thedecoder 50 to supply its power also. In the specific embodiment shown,the pulser circuit 120 is rendered operative to produce a continuoussupply voltage to the receiver circuits 30 until the termination of thecontrol tone. After that predetermined time has lapsed, the pulsercircuit 120 again revcrts to its quiescent operation to produce theseries of pulses on the conductor 136, intermittently to render thereceiver circuits 30 operative.

Also, the decoder St) produces, on the conductor 108, a delayed controlsignal when the RF signal contains the control tone to which the decoder50 is tuned. A timer switch 140, to which the conductor 108 isconnected, produces an enabling signal on the conductor 150 whichrenders the audio amplier 160 operative to amplify the audio signalspresent on the conductor 42. The audio signals are then coupled to thespeaker 180 which converts them into sound waves. Since the decoder. inthe embodiment shown, renders the pulser operative to provide acontinuous supply voltage only for the duration of the control tone,there is provided a feedback circuit 15S connected via a conductor 150from the timer switch 140 through a conductor 158 to the pulser circuit120. The timer switch 14|] may be set to maintain the pulser circuit 120operative to provide a continuous supply voltage for any desired timeinterval even after termination of the control tone, so that thereceiver circuits are operative to relay all lill Ll l] (itl Tll

the audio information contained in the RF signals to a utilizationcircuit such as the audio amplifier 160.

There is illustrated in FIG. 2 of the drawings, a more complete diagramof the communication receiver 2t). The carrier signal is picked up bythe antenna 21 and is conveyed to the input of a radio frequency amplier31. The output of the radio frequency amplifier 31 is applied by aconductor 32 as one of the inputs to the mixer 35, the usual localoscillator 33 being provided and having the output thereof connected bya conductor 34 as a second input to the mixer 35. The intermediatefrequency (IF) signal which is the output of the mixer 35 is applied bya conductor 36 as the input to the IF amplifier 37. the output of whichis transmitted by the conductor 38 to the input of a limiter 39. Theoutput of the limiter 39 appears on a conductor 40 and is the input tothe discriminator 41, the output of the discriminator being a compositedemodulated signal appearing on the conductor 42. The compositedemodulated signal includes audio signals for coupling via conductor 42to the audio amplifier 160 tFlG. l).

The composite demodulated signal present on the conductor 42 alsoincludes a control tone which is applied to a decoder 50, the decoderincluding an amplitier 51 connected to the conductor 42 and having itsoutput coupled via a conductor 52 to a tone filter 53 by a capacitor 54,the tone tilter 53 including a capacitor 55 coupled in parallel with aninductor 56. The filter 53 is coupled between ground and the junction ofa capacitor 71 and the capacitor 54. The decoder 50 further comprises areference circuit 6() including an input capacitor 61 connected to theconductor 52 and a diode 62 connected t0 ground. There is also provideda diode 63 connected between the junction of the capacitor 61 and thediode 62 to a filtering network comprising a resistor 64 and a capacitor65 coupled in parallel to ground. The decoder 50 further includes arectifying circuit having a pair of diodes 72 and 73 coupled in seriesfrom the anode of the diode 63, the capacitor 7l being coupled to thejunction of the diodes 72 and 73. The rectifying circuit also includes aresistor 74 and a capacitor 75 connected in parallel from the` cathodeof the diode 73 to ground to provide filtering for the rectifiedvoltage.

The amplified signal, containing the control tone and voice, on theconductor 52 will be rectified in the reference circuit 60 and will beliltered thereby to provide a reference voltage applied to the anode ofthe diode 72. If the signal on the conductor 52 includes the controltone to which the tilter S3 is series resonant with the capacitor 54,the control tone at an increased amplitude, will appear at the cathodeof the diode 72. In order to provide an output from the diode 73, thecontrol tone appearing at the cathode of the diode 72 must have apeak-to-peak value in excess of the reference voltage on the anode ofthe diode 72, before the diode 72 will conduct to provide an output. Ineffect there is provided a filter 53 followed by a voltage doublercircuit (the tone rectifier circuit which is biased in such a way thatthere is no DC output voltage from the diode 73 until the referencevoltage on the anode of the diode 72 is exceeded. Thus, the bandwidthover which the tone produces a DC output voltage can be readilycontrolled by controlling the relationship between the filter outputvoltage and the reference voltage. With the particular filter shown, forexample, the capacitor 54 may be increased in value to produce a greatertone output and consequently a wider bandwidth or lessened in value tosimilarly produce a narrower bandwidth.

The decoder 50 further includes an electronic switch 80 comprised of anNPN transistor 8l having its emitter grounded and its collector coupledthrough a resistor 82 to a supply voltage, the base being coupled to thecathode of the diode 73. There is provided a rst output stage consistingof a PNP transistor having its base coupled through a resistor 92 to thecollector of the transistor 91.

The emitter of the transistor 91 is coupled to ground via a resistor 93and is coupled to the source of supply voltage by a resistor 94, wherebythe resistors 93 and 94 function as a voltage divider to provide areference voltage on the emitter of the transistor 91. The collector ofthe transistor 91 is coupled by a resistor 95 to the lirst outputconductor 96. There is also provided a second output stage 105 whichincludes a PNP transistor 106 having its base coupled by way of aresistor 101 to the collector of the transistor 81 and having itsemitter coupled to the junction of the resistors 93 and 94. A capacitor102 is coupled between the source of supply voltage and the base of thetransistor 106, the resistor 101 and the capacitor 102 defining a timedelay network 100 as will be explained presently. The collector of thetransistor 106 is coupled by way of a resistor 107 to a second outputconductor 108.

The rectified and filtered DC voltage appearing at the base of thetransistor 81 in the presence of the proper control tone causesconduction of the transistor 81, thus to switch the collector voltagefrom B+ to ground reference potential. This, in turn, causes thetransistor 91 to conduct heavily to provide a positive DC voltage on theconductor 96, which acts as a control signal. The conduction of thetransistor S1 also provides a path for current ow from B+ through thecapacitor 102, the resistor 101 and the collector-emitter junction ofthe transistor 81, thereby to charge the capacitor 102. As soon as thecapacitor 102 has been sufliciently charged, the transistor 106 willbegin to conduct heavily to place a positive voltage on the conductor108, which acts as a second control signal. The charging of thecapacitor 102 effectively delays the time at which the second controlsignal appears on the conductor 108, by an amount depending on the RCtime constant of the capacitor 102 and the resistor 101. Thus, the firstcontrol signal appears on the conductor 96 as soon as the control toneis received, whereas the second control signal does not appear on theconductor 108 for some predetermined time thereafter. The purpose forthe delay network 100 will be explained as the description proceeds.

Referring now to FIG. 3, the lirst control signal on the conductor 96 isapplied to a pulser circuit 120 which includes an astable multivibrator121 in which there is an NPN transistor 122 having its emitter onground, its collector coupled through a resistor 123 to the supplyvoltage, and its base coupled to the cathode of a diode 124, the anodeof which is on ground. The multivibrator 121 also has a second NPNtransistor 125 with its emitter grounded and its base coupled through acapacitor 126 to the collector of the transistor 122. The collector ofthe transistor 125 is coupled to the source of supply voltage by way ofa resistor 127. There is also provided a diode 128 coupled from groundto the base of the transistor 125. Last, the multivibrator 121 includesa feedback capacitor 129 coupled from the collector of the transistor125 back to the base of the transistor 122. The diode 124 provides afast discharge path for the capacitor 129, and the diode 128 provides afast discharge path of the capacitor 126.

The pulser circuit 120 also includes an electronic switch 130 having anNPN transistor 131 with its emitter grounded, its base coupled to theresistor 132 and its collector coupled by way of a resistor 133 to thesource of supply voltage. The switch 130 also includes a PNP transistor134 having its emitter coupled to the source of supply voltage, its basecoupled to the collector of the transistor 131 by way of a resistor 135and its collector coupled to the output conductor 136. Also coupled tothe base of the transistor 131 is the conductor 96.

In operation, the multivibrator 121 serves, by wellknown operation, toproduce a series of pulses having a peak-to-peak value equal to thevalue of the supply voltage. The duration of the pulses is determinedprimarily by the values of the resistor 123 and the capacitor 126; andthe interval between successive pulses is determined primarily by thevalues of the resistor 127 and the capacitor 129. In an operatingcircuit incorporating the present invention, each pulse had a durationon the order oi l5 milliseconds and about 300 milliseconds elapsedbetween successive pulses. The series of pulses is applied to theelectronic switch 130, which causes successive conduction of thetransistors 131 and 134 to provide a series of pulses on the conductor136 having a peak-to-peak value equal to the value of the supplyvoltage. The series of pulses are translated along the conductor 136 tothe various elements of the receiver circuits 30, as is most clearlyshown in FIG. 2 wherein olishoots of the conductor 136 are provided forthe RF amplilier 3l, the local oscillator 33, the mixer 35, the IFamplifier 37, the limiter 39 and the discriminator 41. It should beclear that these pulses of supply voltage render operative each elementin the receiver circuits 30 so that it is able to process RF signalsappearing at the antenna 21. Of course` if an RF signal appears at theantenna 21 between pulses. the receiver circuits 30 will not beoperative and that signal will not be processed.

As explained previously, if an RF signal is received at an instant whena pulse is present, the signal will be processed in the receivercircuits 30, with the audio signals being applied along the conductor 4Zto the audio amplitier 160. If the composite signal on the conductor 42contains the control tone to which the lter 53 is tuned. a first controlsignal will appear on the conductor 96 as previously described. Thecontrol signal on the conductor 96 is applied (FIG. 3) to the base ofthe rst transistor 131 in the switch 130 to render the transistor 131conductive. which in turn renders conductive the transistor 134 to placeon the conductor 136 a constant DC voltage equal to the B t supplyvoltage, which is applied back to each element in the receiver circuits30. Now the receiver circuits 30 are in condition to receive and processany RF signals impressed on the antenna 21 for the duration of thecontrol signal on the conductor 96. It should be apparent that once thecontrol signal is removed, the pulser circuit reverts back to itsoriginal state and produces the series of pulses for intermittentlyenergizing the receiver circuits 30. In the embodiment shown. thecontrol signal on the conductor 96 terminates at the same time that thecontrol tone ends.

However, it may be desired to maintain the receiver circuits 30operative for a period of time after termination of the control tone.For that purpose there is provided an electronic switch circuit 140which may either be timed to maintain the pulser circuit 1.20 operativeto generate a continuous supply voltage for a longer duration, or may beof the latching variety in which case the pulser circuit 120 willproduce a continuous supply voltage until some positive act is effectedby the user to interrupt its operation. In the embodiment shown, theswitch 140 is a monostable multivibrator and functions as a timer.

The electronic switch 140 includes an NPN transistor 141` having itsemitter coupled to ground via a resistor 142 and having its base coupledto ground by way of a resistor 143 and a diode 144 coupled in parallel.'There is also provided a PNP transistor 145 having its base connecteddirectly to the collector of the transistor 141, its collector connectedthrough a resistor 147 to ground and its emitter connected to a sourceof supply voltage. a resistor 146 being connected between the base andthe emitter of' the transistor 145. The collector of the transistor 145is coupled by way of a capacitor 148 to the base of the transistor 141.A conductor 150 is coupled from the resistor 147 to a feedback network155 consisting of a resistor 156 and a diode 157 coupled in series. Thecathode of the diode 157 is coupled through a conductor 158 back to thebase of the transistor 131 in the pulser circuit 120. The conductor 150is also coupled to the audio amplifier 160. There is also provided amanually operable switch 151 coupled between the source of supplyvoltage and the base of the transistor 141. Finally, the conductor 108which carries the delayed control signal from the decoder is coupled tothe base of the transistor 141.

In operation, the appearance of the delayed control signal on theconductor 108 causes conduction of the transistor 14| which provides apath for current flow from the source of supply voltage through thcbase-emitter junction of the transistor and the collector-emitterjunction of the transistor 141. This renders the transistor 145 highlyconductive so as to provide current flow through its collcctonemitterjunction and the resistor 14-7 and thereby place the supply voltage onthe conductor 150. The supply voltagc becomes an enabling signal forrendering the audio amplifier 160 operative, as will be explainedpresently.

During the conduction periods of the transistors 141 and 145. currentflows from Bt through the collector'- emitter junction of the transistor145, through the capacitor 148 and through the base-emitter' junction ofthe transistor 141 to charge the capacitor 118. Accordingly, when thecontrol signal on the conductor 108 is removed by virtue of the controttone terminating, the transistor 141 remains conductor as the capacitor148 continues to charge through the base-emitter junction of thetransistor 141 and the resistors 142 and 143. Of course. the conductionof the transistor 141 maintains the transistor 145 conductive tomaintain the enabling voltage on the conductor 150 for a time intervaldetermined by the RC time constant ot' the switch circuit 140, that is,the resistors 142 and 143 and the capacitor 148. By selecting the valueof those parts. thc time period that the enabling signal remains on tthe conductor 150 may be controlled.

The audio amplilier includes a tt'st stage of ampliiication consistingof an NPN transistor 161 having its emitter coupled through avolnmecontrol potentiometer 163 and a resistor 162 to ground. Thecollector of the i transistor 161 is coupled to thc source of Supplyvoltage and its base is coupled by a resistor 164 and a capacitor 159 tothe conductor 42. A bias voltage is derived by a resistor 165 and adiode 166 coupled in series from the corductor 150 to the base of thetransistor' 161. A capacitor 1660 filters the DC voltage` on theconductor 151). There is also provided a second stage of amplificationconsisting of an NPN transistor 167 having its erritter on ground. itscollector coupled through a choke 168 to the source of supply volt-.igeand its base coupled through a capacitor 169 to the movable arm of thepotentiometer 163. Bias voltage for this transistor is supplied by aresistor 170 coupled between its base and the conductor 150. There isalso provided a third stage of amplification consisting of a pair ofcomplementary symmetry transistors v171 and 172, the base voltage forthese transistors being supplied by a voltage divider consisting ofresistors 173 and 174 connected between the conductor 150 and ground.The collector of the transistor 167 is coupled through a capacitor 175to the connected-together bases of the transistors 171 and |72. Thecoitectors of the transistors 171 and 172 are respectively coupled toB't and ground. The emitters of the transistors 171 and 172 areconnected together and through a capacitor 176 to an annunciator such asthe speaker 180. A feedback capacitor 178 reduces crossover distortionby providing negative feedback.

In operation, the enabling signal appearing on the conductor 150 inresponse to a control tone, is applied through the diode 166 and theresistor 165 to establish a. positive voltage on the base of thetransistor 161 and thereby render it conductive. The diode 166 preventsthe audio signals on the conductor 42 from being applied to theelectronic switch l-tt) by way of the conductor 150. Similarly. theenabling signal on the conductor 150 is applied through the resistor 170to the transistor 167 to render it conductive also. Also, the enablingsignal provides a bias voltage across the resislor 174 for thetransistors 17| and |72. ln this condition. audio signals on theconductor 42 will be amplified by the transistor 161, then (ill by thetransistor 167 and power amplified by the transistors 171 and 172. Thepotentionmeter 163 functions as a volume control and is accessible tothe user of the receiver. Of course, without the enabling signal on theconductor 150, none of the transistors 161, 167, 171. or 172 arcoperative to amplify the audio signals on the conductor 42. It is thusdesirable that the RC time constant in the electronic switch circuit 140be selected to be long enough to maintain the audio amplifier 160operative for the duration of the audio information. However, if theaudio information extends beyond the time that the electronic switch14() opens, the user can close the manual switch 151 which provides theenabling signal on the conductor 150. In the circuit shown in FIG. 3,the audio arnplier 160 may be viewed as a utilization circuit whichutilizes the enabling signal appearing on the conductor 15E. The speaker180 may be viewed as an annuciator for the audio signal developed in theaudio amplier 160.

Recapitulating, the pulser circuit 120 produces a series of pulses onthe conductor 136 which are used intermittently to provide supplyvoltage for the various elements in the receiver circuits 30. 1n aparticular' embodiment, the pulso width was I5 milliseconds and the timebetween pulses was 360 milliseconds, or a 4 percent duty cycle. Thismeans that during 96 percent of the time, the communication receiver Z0was drawing essentially no current, and during the other 4 percent ofthe time the receiver was drawing standby current. In standardcommunication receivers, the receiver circuits are maintainedcontinuously operative so that they provide a constant drain on thebattery. Since the receiver 20 receives audio signals intended for it afraction of i percent during the course of a day. the greatest drain onthe battery is the amount of standby current drawn. It can beappreciated that in the case of the standard" receiver, where thereceiver circuits continually draw current, there is a maximum drain onthe battery and a minimum useful life thereof. On the other hand, usingthe 4 percent duty cycle as an example, the receiver circuits 30 aredrawing standby` current only 4 percent of the time, whereby the usefullife of the battery may be increased theoretically by a factor of 25. Ofcourse the pulscr circuit 120 does draw some current, so that the actualincrease in battery life muy be slightly' less than 25 times. Thecircuit disclosed above means that a battery having lesser capabilities,and thus smaller size, can be used. This is most important in theparticular type of receiver to which the invention is particularlyadapted, namely, a portable one. To be portable, the size of thereceiver must be minimized, and, since batteries always consume asubstantial portion of the usable space, it is an important advantage tobe able to reduce the size of the battery without sacrificingperformance of the receiver. As a matter of fact, the size of. thebattery may be reduced and its useful life may be substantiallyincreased at the same time by virtue of the above invention. Also,manufacturers of this type of portable equipment strive always to reducethe Weight of the receiver', another objective which is accomplished bythe above invention due to the smaller size of the batteries. Referenceis made to the graph of FIG. 4, wherein the waveform represents thesignal appearing on the conductor 136 (see FIG. 3) which is the outputof the pulser circuit 120 and consists of a series of pulses 191. Forpurpose of illustration, the duration of each pulse is 15 millisecondsand 360 milliseconds elapses between pulses. Accordingly, the receivercircuits 30 are rendered operative for the duration of each pulse 191and are inoperative between the pulses 191. If an RF signal carrying acontrol tone represented by the waveform 195 is impressed on the antenna21 `during the presence of a pulse 191, it will be detected in thediscriminator 41 and will appear on the conductor S2. If the controltone has the frequency to which the filter circuit 53 is tuned, it willpass into the rectifier circuit 70. The signal on the conductor 52,including the noise thereon, is rectified by the reference circuit 60 toprovide a reference voltage on the anode of the diode 72. If the signal195 exceeds the reference voltage, it will be rectied in the rectiercircuit 70 and ampliiied by the transistors 81 and 91 to provide acontrol signal on the conductor 96. This causes the electronic switch130 to close and provide a continuous supply voltage, which is indicatedby the numeral 192 of the waveform 190. The continuous supply voltage isapplied to the receiver circuits 30 to cause the control signal 196 toappear on the conductor 96. It should be noted that, although thecontrol tone commenced at t1, it would not be processed by the receivercircuits 30 since at t1 the supply voltage was not being applied thereo.At t2, however, a puise 191 has commenced to render the receivercircuits 30 operative to translate the RF signal and detect the controltone 195 therein. There is a short delay of perhaps 8 milliseconds forthe decoder 50 to respond so that at t3 the control signal 196commences, and terminates at t5 with the termination of the controltone. Without more, the continuous supply voltage 192 would alsoterminate at this time. The output of the decoder 50 on the conductor108 is shown as a waveform 197 and, as can be seen, it has the sameappearance as the waveform 196, but delayed in time so that it commencesat t4. This is, of course, due to the delay provided by the network 100in the decoder 50. The control signal on the conductor 108 causes theswitching circuit 140 to provide on the conductor 150 an enabling signalrepresented by the waveform 198, commencing at t4. One leg of theconductor 150 is coupled through the feed back network 155 to theelectronic switch 130 in the puiser circuit 120 to maintain the switchclosed and continue to p-rovide the continuous voltage supply 192 inspite of the termination of the control tone at t5. The continuoussupply voltage on the conductor 136 will be present until t8 which isdetermined by the time constant in the electronic switch circuit 140. Itis, therefore, apparent that the receiver circuits 30 are operative totranslate audio information via the conductor 42 to the audio amplifier160 for the duration of t2 to t6. If the user finds that additionalaudio information is still being received, he can close the manualoverride switch 151 to maintain the continuous supply voltage 192 beyondt6.

The conductor 150 also couples the enabling signal to the audioamplifier 160 to render the same operative to amplify the audio signalson the conductor 42. The period of conduction of the audio amplifier 160is during the period t4 to t6. However if the audio information is stillbeing received at t8, the manual override switch 151 can be closed tomaintain the audio amplifier 160 operative beyond t6. In one embodimentof the invention, the duration of t4 to te was ten seconds, although anytime shorter or longer than that is easily attained.

The ldelay provided by the network 100 in the decoder 50 is to minimizethe possibility of the receiver responding to a false signal,particularly noise. Of course noise contains a wide spectrum of signalsincluding the signal to which the filter circuit 53 in the decoder 50responds. Accordingly, such a noise signal can provide a control signalon the condu-ctor 96 to render the puiser 120 operative to produce acontinuous supply voltage for the receiver circuits 30. The control tonein the noise is necessarily very short in duration so that it isunlikely that the receiver circuits will be on for more than a couple ofmilliseconds or so, whereby no increase in current drain occurs. Such asignal would not yield a control signal on the conductor 108 because ofthe delay in the network 100. Accordingly, in the presence of noise orother extraneous signals, no enabling signal is provided on theconductor 150, whereby no voltage is fed back to the puiser circuit 120to lengthen the duration of the continuous supply voltage and wherebythe audio amplifier 160 is not rendered conductive.

To insure that the control tone will operate the receiver in the mannerdescribed, the duration thereof should be longer than the lapsed timeinterval between successive pulses 191 by an amount at least equal tothe turn on delay period tg-tt. So, in the example given, if the timebetween pulses is 360 milliseconds, then a control tone that lasts for400 milliseconds plus the turn on delay period tft., will necessarily bepresent during the occurrence of a puise 191 and also provide forvariations in the elements of the receiver.

An important feature of the invention is the fact that it responds to acontrol tone and will not respond to carrier signals alone. If the RFsignal received by the receiver 20 does not contain a control tone atthe frequency to which the filter circuit 53 is tuned, the puisercircuit 120 will not provide a continuous supply voltage, nor will theaudio amplilier 160 be turned on. This is particularly important when itis considered how necessary it is today to make optimum use of thefrequency spectrum. If the receiver were to be rendered operative solelyby an RF signal of the proper frequency, it would be turned on many,many times during the day, even though it contained information for theuser of that receiver perhaps two or three times. On the other hand, thecommunication receiver 20 described will draw current only during thepresence of the proper carrier containing the proper control tone, thusto energize the receiver 20 only during the two or three times a daythat the receiver is called.

In a typical operating example of the circuits shown in FIGS. 2 and 3,the various components thereof had the following values: capacitors 54and 55 and the inductor 56 had values determined by the frequency towhich the decoder 50 was to respond; the capacitor 61, 0.02 microfarads;the resistor 64, l megohrn; the capacitor 65, 0.02 microfarads; theresistor 74, 5 megohms; the capacitor 75, 0.01 microfarad; the resistor82, 1 megohm; the resistors 92 and 101. l megohm; the resistors 93 and94, 100 kilohms; the resistor 95, 220 kilohms; the resistor 107,kilohms; the resistor 123, 220 kilohms; the capacitor 126, 0.1 farad;the resistor 127, 220 kilohms; the capacitor 129, 0.02 farads; theresistors 132 and 133, 470 kilohms; the resistor 135, 4.7 kilohms; theresistor 146, 2.2 megohms; the resistor 147, 100 kilohms; the capacitor148, 1.5 microfarads; the resistor 143, 2.2 megohms: the resistor [143]142, 22 kilohms; the resistor 156, 220 kilohms; the capacitor 159, 82()picofarads; the resistor 164, 1 megohm; the resistor 165, 220 kilohms;the capacitor [166] I66a, 1.5 microfarads; the potentiometer 163, Oto l0kilohms.

Referring now to FIG. 5 of the drawings, there is illustrated a secondembodiment of the present invention wherein a sequence of four tones isrequired to actuate the communication receiver which is designated bythe numeral 220. The receiver 220 includes an antenna 221 for receivingRF signals, and receiver circuits 230 comprising the same elements asthe receiver circuits 30 shown in FIG. 2. In the interest of brevity.further detailed description of the various elements of the receivercircuits 230 will be omitted. Appearing on the conductor 231, the outputof the receiver circuits 230, is a composite demodulated signalincluding the control tones and intelligence if any. In the particularembodiment shown in FIG. 5, the communication receiver 220 has noprovision for audio circuitry but rather is a paging device.Accordingly, the conductor 231 will not have any intelligence (i.e.,voice) thereon. There is provided a puiser circuit 500 which isconstructed and operates similarly to the puiser circuit in the firstembodiment. The puiser circuit 500 produces a series of pulses on theconductor 516, which is coupled back to the receiver circuits 230 toprovide the supply voltage therefor. During the presence of the pulses,the receiver circuits 230 are operative to process and detect RF signalsimpressed on the antenna 221; whereas between successive pulses, the

receiver circuits 230 are inoperative and any signals on the antenna 221will not pass through to the decoder.

A decoder 240 is coupled to the conductor 231 and if the control tonesare at the frequencies to which the decoder 240 is timed, a controlsignal will be developed on the conductor 473 for application to thepulser circuit 500. The control signal on the conductor 473 commencesessentially at the same time as the inception of the first control tonein the series of control tones, the control signal causing the pulsercircuit 500 to furnish a continuous supply voltage for a predeterminedinterval on the conductor S16, which supply voltage renders the receivercircuit 230 continuously operative for that interval to process anddetect RF signals on the aritenna 221. Upon termination of the lastcontrol tone in the series, the control signal on the conductor [416]516 is removed and the pulser circuit 500 again produces a series ofpulses for intermittent operation of the receiver circuits 230. A secondoutput of the decoder 240 appears on the conductor 465 and carries asecond control signal that commences essentially with the reception ofthe last control tone in the series of control tones, assuming theprevious ones have been received in the proper order. The control signalon the conductor 465 terminates with the termination of the last controltone.

The control signal on the conductor 465 is applied to a timer switchcircuit 370 which, in turn, energizes a utilization circuit such as theoscillator 540. The series of pulses from the pulser circuit S00 is alsoapplied to the oscillator 540. and, in the presence of both signals. a

pulsating oscillatory signal is applied to an annunciator such as thespeaker 545 which generates a series of bursts of alerting tones.

The control signal on the conductor 465 is also applied to a latchingswitch circuit 570 which, in turn. energizes a utilization circuit suchas the lamp control circuit 590. Also applied to the lamp controlcircuit 590 is the series of pulses on the conductor 516. In the`presence of both the series of pulses and the enabling signal from thelatching switch circuit 570, an annunciator such as the lamp 600 blinkson and off at a rate determined by the series of pulses. Another outputfrom the latch switching circuit on the conductor 582 is applied to apulse extender circuit 610 which, upon termination of the last controltone, lengthens the pulses developed by the pulser circuit 500 toincrease the duration of the bursts from the speaker S45 and to increasethe on-time of the lamp 600. After expiration of a predetermined time.the timer switch circuit 370 ceases to provide the enabling signal onthe conductor 530 and the bursts of audio cease.

When the user operates a manual switch in the latching switch circuit570, the lamp 600 becomes extinguished and the pulser circuit 500reverts to producing pulses of shorter duration.

The output from the receiver circuits on the conductor 231 is applied tothe decoder 240 which is shown in block form in FIG. 6. The decoder 240is adapted to respond to a series of four control tones received in apredetermined order. The signal on the conductor 231 is applied to apair of tone control channels, the lower tone control channel includinga special tapped filter 241 of a construction to be describedhereinafter. If the filter 241 is tuned to the frequency of the firstcontrol tone on the conductor 231, it will pass to the conductor 246 andbe applied to a rectifier 260. The control tones and any noise on theconductor 231 are also applied to a reference circuit 270 which providesa reference voltage on the conductor 275. lf the first control tone onthe conductor 246 exceeds the reference voltage on the conductor 275,the rectifier 260 will operate to rectify the first control tone andprovide a filtered DC voltage on the conductor 266. The DC voltage isapplied to :in electronic switch 280 so as to power amplify the voltageand apply it on a conductor 285 as one input to an AND circuit 290, Asecond input for the AND circuit, on

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a conductor 426, is derived from an inverter 420. If both inputs arepresent, an output voltage on the conductor 294 will result, whichvoltage is applied to a timer 300. Upon termination of the nrst controltone, a DC voltage appears on the conductor 303 and persists for aduration dependent on the setting of the timer 300. The voltage on theconduc tor 303 is coupled to an electronic switch 310 which provides aDC voltage pulse on its output conductor 314. The 'voltage on theconductor 314 is coupled to a filter 341 in the second control tonechannel and tunes the same to receive thc second control tone present onthe conductor 2311. lf the second control tone on the conductor 231appears immediately and is at tlie frequency to which the filter 341 isnow brieiiy tuned, it will pass to the conductor 346 and will be appliedto a rectifier 360. lf the second control tone on the conductor 346exceeds the reference voltage on the conductor 27S, the rectier 360 willoperate to rectify the second control tone and provide a filtered DCvoltage on the conductor 366. The DC voltage is applied to an electronicswitch 380 so as to power amplify the voltage and apply it on aconductor 385 as one input to an AND circuit 390. The second input forthe AND circuit 390 is the voltage on the conductor 314. Thus, if thefirst control tone was received and has terminated so as to provide avoltage pulse on the conductor 314, and the second control tone is beingreceived while that pulse is present to provide a DC voltage on theconductor 385, the AND circuit 390 will operate to produce a DC outputvoltage on the conductor 394. This voltage is applied to a timer 400which provides a DC voltage on the conductor 403 upon termination of thefirst control tone, persisting for a duration dependent on the settingof the timer 400. This voilage is applied to an electronic switch 41|)which produces a DC voltage on the conductor 414 for application to theinverter 42() so as to place the same in its other stable condition. Theresulting output from the inverter 420 on the conductor 426 is coupledhack to one input of the filter 241 which causes the lilter 241 to nolonger be tuned to the first control tone. Simultaneously, the voltageon the conductor 414 is applied to another input of the filter 214 toretune the same to respond to the third control tone. Finally, thevoltage on the conductor 414 is applied as a first input to an ANDcircuit 430.

If the proper third control tone is received on the coriductor 231, thelter 241 will pass the third control tone to the rectiticr 260. If thethird control tone exceeds the reference voltage on the conductor 27S,it actuates the electronic switch 280 to provide one input to the ANDcircuit 290. However, the inverter is in its second stable condition sothat a second input to energize the AND circuit 290 is lacking. Thethird control tone, in addition, provides a second input to the ANDcircuit 430. With both inputs to the AND circuit 430, a potential isdeveloped on the conductor 438 which is applied back to the input of theelectronic switch 41() to hold it in the active condition as long as thethird tone is received. A second output of the AND circuit 430 on theconductor 437 is applied to a timer 440. Upon termination of the thirdcontrol tone, a DC voltage appears on the conductor 443 and persists fora duration dependent on the setting of the timer 440. This voltage isapplied to an electronic switch 45t) which produces a DC voltage on theconductor 454. This output voltage is fed back lo the tiltcr 341 toretune the same so as to be operative to receive the fourth controltone. The signal on the conductor 454 is also applied as one of theinputs to an AN D circuit 466.

Assuming that the proper fourth tone in the sequence of tones is nowreceived, there will be an output from the filter 341 which will berectified in the rectifier 360 to provide a DC voltage. This voltageoperates the electronic switch 380 and provides a second input, on theconductor 385, to the AND circuit 460. In the presence of both inputs,the AND circuit 460 provides a control signal on the conductor 465. Ahold-on potential is applied from the AND circuit 460 on the conductor466 to the input of the 1 3 electronic switch 450 to hold the latter inits active condition as long as the fourth tone is being received.

Also provided in the decoder 240 is a pulser control circuit 470 havinga pair of inputs respectively coupled to the conductors 285 and 385. Thepulser control circuit 470l is operative to provide on its outputconductor 473 a control signal commencing with inception of the firstcontrol tone, it being pointed out that a voltage appears on theconductor 285 throughout the first and third tones and a voltage appearson the conductor 385 throughout the second and fourth tones, so that avoltage is continually being supplied to the pulse control circuit 470to cause a continuous control voltage to appear on the conductor 473 forthe duration of the control tones.

Referring now to FIGS. 7 and 8 of the drawings, there are illustratedfurther details of the decoder 240. The lter 241 includes an inductor242 having associated therewith a magnetic core 243, at least a portionof the core 243 being movable and adjustable, whereby the inductor 242can be slug tuned. The inductor 242 is connected through a capacitor 245to the conductor 231, and a capacitor 244 is coupled from the top of theinductor 242 to ground. The ouput from the filter 241 appears on aconductor 246. The inductor 242 has a plurality of taps thereon, two ofwhich are identified by the numerals 247 and 248. Associated withselected ones of the taps are two NPN transistors 250 and 253. Aresistor 249 is coupled between the base of the transistor 250 and theconductor 426. The transistor 250 has a collector connected to the tap248 on the inductor 242, while the emitter is connected to groundpotential. A resistor 254 is coupled between the base of the transistor253 and the conductor 414. The transistor 253 has a collector connectedto the tap 247 on the inductor 242, while the emitter is connected toground potential.

The decoder 240 also includes an inverter 420 including a PNP transistor421, the base of which is coupled through a resistor 422 to ground andthrough a diode 423l and a resistor 424 to the conductor 414. A sourceof B+ supply voltage is coupled to the emitter of the transistor 421through a diode 425. In its quiescent condition, the transistor 421 isheavily conductive so that the supply voltage appears on the conductor426 to render the transistor 250 in the filter 241 heavily conductive,thereby effectively to ground the tap 248 on the inductor 242. In thiscondition, there is defined a parallel resonant circuit in the filter241, composed of the capacitor 244 coupled across the top half of theinductor 242. If the first control tone on the conductor 231 is at thefrequency to which the filter 241 is now tuned, the control tone, at anincreased amplitude, will appear on the conductor 246. It should benoted that, at this time, the transistor 253 is nonconductive.

The control tone, together with the noise on the conductor 231, isapplied to a reference circuit 270 which is constructed like thereference circuit 60 in the first embodiment, and, in the interest ofbrevity, no further explanation will be provided, except that areferenec voltage is provided on the conductor 275 proportional inamplitude to the control tones and noise on the conductor 231. The firstcontrol tone on the conductor 246 is applied to a rectifier 260 whichhas the same construction as the rectifier 70 in the first embodiment,and, again in the interest of brevity, no further explanation will bemade, except that a DC voltage will be present on the conductor 266 ifthe control tone on the conductor 246 exceeds the reference voltage onthe conductor 275.

The next stage is an electronic switch 280 consisting of a pair ofcascaded NP'N transistors 281 and 283, having their collectors coupledto a DC voltage supply respectively via resistors 282 and 284. The DCvoltage on the conductor 266 will cause the transistors 281 and 283 toconduct heavily, so as effectively to ground the collector of thetransistor 283.

The next stage is an AND circuit 290 including a PNP transistor 292having a base coupled by way of a resistor 291 to the conductor 285. Theemitter of the transistor 292 is coupled by way of a diode 293 to theconductor 426, and the collector is coupled to ground through a resistor301. There are two inputs to the AND circuit 290 from the conductors 285and 426. If the conductor 285 is effectively grounded, which occursthrough the transistor 283 when the rst control tone is present, and ifthe positive voltage appears on the conductor 426, which occurs when theinverter 420 is in its quiescent condition, the transistor 292 becomesheavily conductive to place a positive voltage on the conductor 294. Atimer 300, consisting of the resistor 301 and a capacitor 302 produces anegative DC voltage on the conductor 303 upon termination of thepositive voltage on the conductor 294 which occurs upon termination ofthe first control tone. The next stage is an electronic switch 310 whichincludes a PNP transistor 311 having its emitter coupled to the sourceof supply voltage, having its base coupled thereto through a resistor312 and a diode 313, and having its collector coupled to the conductor314. While the first control tone is being received, the capacitor 302is being charged through the diode 313 and the transistor 311 is notconductive. However, upon termination of the first control tone, thecapacitor 302 discharges through the resistor 301 to render thetransistor 3111 heavily conductive to place the supply voltage on theconductor 314. This voltage persists for a duration determined by the RCtime constant of the timer 300. The positive voltage on the conductor314 is applied as one input to the AND circuit 390 and as an input tothe second filter' 341.

The filter 341 includes an inductor 342 having associated therewith amagnetic core 343, at least a portion ofthe core 343 being movable andadjustable, whereby the inductor 342 can be slug tuned. The inductor 342is connected through a capacitor 345 to the conductor 231, and acapacitor 344 is coupled from the bottom of the inductor 342 to ground.The output from the filter 341 appears on a conductor 346. The inductor342 has a plurality of taps thereon, two of which are identified by thenumerals 347 and 348. Associated with selected ones of the taps are twoNPN transistors 350 and 353. A resistor 349 is coupled between the baseof the transistor 350 and the conductor 314. The transistor 350 has acollector connected to the tap 348 on the inductor 342, while theemitter is connected to ground potential. A resistor 354 is coupledbetween the base of the transistor 353 and the conductor 454. Thetransistor 353 has a collector connected to the tap 347 on the inductor342 while the emitter is connected to ground potential.

The positive supply voltage on the conductor 314 developed during thepresence of the first control tone renders the transistor 350 in thefilter 341 heavily conductive thereby effectively to ground the tap 348on the inductor 342. In this condition, there is defined a parallelresonant circuit composed of the capacitor 344 coupled across the bottomportion of the inductor 342. If the second control tone in the series ofcontrol tones on the conductor 231 is at the frequency to which thefilter 34.1 is then tuned, the control tone, at au increased amplitude,will appear on the conductor 346. It should be noted that at this timethe transistor 353 is nonconductive.

The second control tone on the conductor 346 is applied to a rectier 360which is constructed like the rectifier 260. rectified DC voltage willappear on the conductor 366 if the second control tone exceeds thereference voltage on thc conductor 275.

The next stage is an electronic switch 380 consisting of a pair of NPNtransistors 381 and 383 coupled in cascade, and respectively havingtheir collectors coupled to the source of supply voltage by way ofresistors 382 and 384. The rectified DC voltage on the conductor 366causes the transistors 381 and 383 to conduct heavily, therebyeffectively grounding the collector of the transistor 383.

The next stage is an AND circuit 390 comprised of an PNP transistor 391having its base coupled to the conduc- 15 tor 385 by the resistor 392.The collector of the transistor 391 is coupled to ground through aresistor 402. There is further provided an NPN transistor 393 having itsbuse coupled to the emitter of the transistor 391. and its emittercoupled to the conductor 385 by a resistor 396. The junction ol the baseof the transistor 393 and the emitter ol the transistor 391 is coupledto the conductor 314. 'the two inputs for the AND circuit 390 are on theconductors 385 and 314. lt will be remembered, that a positive voltageappeared on the conductor 314 after termination ol` the lrst controltone. which positive voltage. in coniunction with the grounding of theconductor 335, by irrimediate reception of the second tone causes bothtransistors 391 and 393 to conduct heavily` The collector of thetransistor 393 is coupled by way of a conductor 395 back to theconductor 303. The heavy conduction of the transistor 393 permitscurrent to llow from Bl' through the haseemitter junction of thetransistor 311, through the collector-emitter junction of the transistor393.y and through the collector-emitter junction of the transistor 383.to maintain the transistor 311 Conductive tor the duration of the secondcontrol tone. As long as the transistor 311 is conductive. one input tothe AND circuit 390 is provided and. as long as the second control toneis present. the second input to the AND circuit 390 is provided. Thus nDC voltage will be present on the conductor 385 for the duration of thesecond control tone. A second output Ifrom the AND circuit 390 on theconductor 394 is derived vfrom the collector of the transistor 391. Atimer 400, consisting of the resistor 402 and a capacitor 401 produces anegative DC voltage on the conductor 403 upon termination of thepositive voltage on the conductor 394 which occurs upon termination ofthe second control tone. The next stage is an electronic switch 410.which includes a PNP transistor' 411 having its emitter coupled to thesource of supply voltage and having its base coupled to said source hyway of a resistor 412 and a diode 413. While the second control tone isheing received, the capacitor 401 is being rapidly charged through thediode 413 and the transistor 411 is not conductive. However. upontermination of thc second control tone. the capacitor 401 dischargesthrough the resistor 40. to render the transistor 411 heavily conductiveto place the supply voltage on the conductor 414. This voltage persistsfor a duration determined by the RC time constant ofthe timer 400.

The positive voltage on the conductor 414 is coupled to the invertercircuit 420 to render the transistor 421 nonconductive. which in turnrenders nonconductive the transistor 250 of the filter 241. Also. the`conductor 414 applies a positive voltage to the base of the transistor253 to render it heavily conductive, thereby effectively placing thecapacitor 244 across the top portion of the coil 242. If the thirdcontrol tone in the series of control tones on the conductor 231 has afrequency to which that resonant circuit is tuned. the resonant circuitwill develop a voltage on the conductor 246 which will be rectitied hythe rectier 260 to provide a DC voltage to operate the switch 280. theoutput of which is applied as one input to the AND circuit 290. Sincethe voltage on the conductor 426 is no longer positive, the AND circuit291! will not `operate in spite of the presence of the voltage on theconductor 28S. The DC voltage is also applied to an AND circuit 430, andAND circuit 430 including an NPN transistor 431 having its base coupledto the emitter of a PNP transistor 432. The base of the transistor 432is coupled to the conductor 285 by means of a resistor 434. and theemitter on the transistor 431 is coupled hy a resistor 435 to theconductor 285. The conductor 414 is coupled to the junction of the baseof the transistor 431 and the cmitt-er of the transistor 432. Thegrounded condition of the conductor 285, resulting from the presence ofthe third control tone, and the plus voltage on the conductor 414.rcsulting from the cessation of the second control tone. cause thetransistors 431 and 432 to conduct heavily. The collll [all

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lector of the transistor 431 is effectively grounded which provit'tei1 apath for current ow through the base-emitter junction ol the transistor411 to cause the transistor to continue to conduct heavily despiteinterruption of the second control tone. Thus, the conductor 438 is afeedback path to maintain conductive the transistor 411 for the durationof thc third control tone. The heavy conduction of the transistor 432effectively places a positive voltage on the conductor 437 which isapplied to a timer 44|) consisting ol a resistor 441 to ground and aseries capacitor 442, The timer 440 produces a negative DC voltage onthe conductor 443 upon termination of the positive voltage on theconductor 437 which occurs upon termination of the third control tone.The next stage is the electronic switch 450 comprised of a PNPtransistor 451 having its hase coupled to the conductor 443, and itsemitter coupled to B l There is also provided a resistor 452 and a diode453 coupled in parallel between the base of the transistor 451 and thevoltage supply source. While the third control tone is being received,the capacitor 442 is being rapidly charged through the diode 453 and thetransistor 451 is not conductive. However, upon termination of the thirdcontrol tone, the capacitor 442 discharges through thc resistor 441 torender the transistor 451 heavily conductive to place the supply voltageon the conductor 454. This voltage persists for a duration determined bythe RC time constant of the timer 440. The next stage is an AND circuit460 including an NPN transistor 462 having its base coupled to theemittter of a PNP Iransistor 467. The base of the transistor 467 iscoupled hy way of a resistor 463 to the conductor 385. The base of thetransistor' 462 and the emitter of the transistor 467 are connectedtogether and to the conductor 454.

When the transistor 451 conducts heavily in response to the terminationof the third control tone, the supply voltage is eti'ectively on theconductor 454 which is coupled haci; to the filter 341 to cause heavyConduction of the transistor 353 thus eifectively to place the capacitor344 across a dillerent. greater portion of the inductor 342. It shouldbe noted that the transistor 350 is now nonconductive since thetransistor 311 became nonconduetive upon termination of the thirdcontrol tone. If the fourth control tone in thc series of control toneson the conductor 231 has a frequency corresponding to the resonantfrequency of the resonant circuit. it will be :ectied by the rectier 360and switch the electronic switch 380, thereby grounding the conductor385 which results in the grounding of the junction between the resistors463 and 464 in the AND circuit 460. The concurrent grounding of theconductor 385 and the presence of the supply voltage on the conductor454 cause the transistor 467 to conduct heavily and provide on itscollector and thus the conductor 465i. a positive control voltagesubstantially equal to the B+ supply voltage. Also, the heavy conductionof the transistor 462 effectively grounds its collector, and thus theconductor 466, so that the transistor 451 remains conductive until thefourth control tone terminates thereby removing the ground from 385.

T hc decoder 240 also includes a pulser control circuit 470 including aPNP transistor 474 having its emitter coupled to the supply voltage andits base coupled to the input conductors 285 and 385 respectively by theresistors 475 and 476. The collector of the transistor 474 is coupledthrough a resistor 477 and a. diode 478 to the conductor 473. There isalso provided an integrating network 479 connected to the collector ofthe transistor 474 and including a parallel resistor and capacitorarrangement. lt will be remembered that the conductor 285 is effectivelygrounded upon inception of the first control tone and stays groundedthroughout the first control tone and also throughout the third controltone. Similarly, the conductor 385 is grounded throughout the second andfourth control tones. whereby the transistor 474 is biased on frominception of the first control torte until termina- 17 tion of the lastcontrol tone and is effectively saturated so as to provide on theconductor 473 a second control signal equal to the supply voltage.

Accordingly, the decoder 240 provides two control signais: a firstcontrol signal on the conductor 465 that does not appear until thecommencement of the fourth tone in the series of four tones to which thedecoder 240 is set, and a second control signal also equal to the supplyvoltage, which is derived on the conductor 473, but appears concurrentlywith the appearance of the first control tone.

Referring now to FIG. 9, the signal on the conductor 473 is applied to apulser circuit 500 which includes an astable multivibrator 501 in whichthere is an NPN transistor 502 having its emitter on ground itscollector coupled through a resistor 503 to a supply voltage, and itsbase coupled to the cathode of a diode 504, the anode of which is onground. The multivibrator 501 also has a second NlPN transistor 505 withits emitter grounded and having its base coupled through a capacitor 506to the collector of the transistor 502. The collector of the transistor505 is coupled to the source of supply voltage by way of the resistor507. There is also provided a diode 508 coupled from ground to the baseof the transistor 505. Lastly, the multivibrator 501 includes a feedbackcapacitor 509 coupled from the collector of the transistor 505 back tothe base of the transistor 502.

The pulser circuit 500 also includes an electronic switch 510 having anNPN transistor 511 with its emitter grounded and its base coupled to theresistor 512 and its collector coupled by way of a resistor 513 to thesource of supply voltage. The switch 510 also includes a PNP transistor[516] 514 having its emitter coupled to the source of supply voltage,its base coupled to the collector of the transistor 511 by way of aresistor S and its collector coupled to the conductor 516. Also coupledto the base of the transistor 511 is the conductor 473.

In operation, the multivibrator 501 serves to produce a series of pulseshaving a peak-to-peak value equal to the value of the supply voltage.The duration of the pulses is determined primarily by the values of theresistor 503 and the capacitor 506, and the interval between successivepulses is determined primarily by the values of the resistor 507 and thecapacitor 509. In an operating circuit incorporating the presentinvention, each pulse had a duration on the order of lS-milliseconds andabout 360 milliseconds elapsed between successive pulses. The series ofpulses is applied to the switch 510 through the transistors 511 and 514to provide a series of pulses on the conductor 516 having a peak-to-peakvalue equal to the value of the supply voltage. The series of pulses aretranslated along the conductor 516 to the various elements of thereceiver circuits 230 (see FIG 5). It should be clear that these pulsesof supply voltage render operative each element in the receiver circuits230, so that they are able to process RF signals appearing at theantenna 221. Of course, if an RF signal appears at the antenna 221between pulses, the receiver circuits 230 will not be operative and thatsignal will not be processed.

If an RF signal is received at an instant when a pulse is present, thesignal will be processed in the receiver circuits 230. If the compositesignal on the conductor 231 contains the sequence of control tones towhich the decoder 240 is tuned, a control signal will appear on theconductor 473 as previously described upon inception of the firstcontrol tone throughout the four control tones. This control signal onthe conductor 473 is applied (FIG. 9) to the base of the firsttransistor 511 in the switch 510 to render the transistor S11conductive, which, in turn, renders conductive the transistor 514 toplace on the conductor 516 a constant DC voltage equal to the B+ supplyvoltage, which is applied back to each element in the receiver circuits230. Now the receiver circuits 230 are in condition to receive andprocess any RF signals impressed on the antenna 221 for the duration ofthe control signal on the conductor 473. It should be apparent that,once the control signal is removed, the pulser circuit 500 reverts backto its original state and produces the series of pulses forintermittently energizing the receiver circuits 230. In the embodimentshown, the control signal on the conductor 473 terminates at the sametime that the last control tone ends.

There is also provided a switch circuit 520 which may either be timed tomaintain the pulser circuit 500 operative to generate a continuoussupply voltage for a longer duration, or may be of the latching variety,in which case, the pulser circuit 500 will produce a continuous supplyvoltage until some positive act is effected by the user to interrupt itsoperation. In the embodiment shown, the switch is a monostablemultivibrator and functions as a timer.

The switch 520 includes an NPN transistor S21 having its emitter coupledto ground via a resistor 522 and having its base coupled to ground byway of a resistor 523 and a diode 524 coupled in parallel. There is alsoprovided a PNP transistor 525 having its base connected directly to thecollector of the transistor 521, its collector connected through aresistor 527 to ground and its emitter connected to a source of supplyvoltage, a resistor 526 being connected between the base and the emitterof the transistor 525. The collector of the transistor 525 is coupled byway of a capacitor 528 to the base of the transistor 521. A conductor530 is coupled to the collector of the transistor 521. The base of thetransistor 521 is coupled to the conductor 465 by way of a diode 532.

In operation, the appearance of the control signal on the conductor 465upon inception of the last control signal causes conduction of thetransistor 521 which provides a path for current ow from the source ofsupply voltage through the base-emitter junction of the transistor 525and the collector-emitter junction of the transistor 521. This rendersthe transistor 525 highly conductive so as to provide current flowthrough the collector-emitter junction of the transistor S25 and theresistor 527 and thereby t0 place an enabling signal on the conductor530.

During the conduction periods of the transistors 521 and 525, currentflows from B+, through the collectoremitter junction of the transistor525, through the capacitor 52S, and through the base-emitter junction ofthe transistor 521 to charge the capacitor 528. Accordingly, when thecontrol signal on the conductor 465 is removed by virtue of the lastcontrol tone terminating, the transistor 521 remains conductive becausethe capacitor S28 is still being charged through the base-emitterjunction of the transistor 521 and the resistors 522 and 523. Of course,the conduction of the transistor 521 maintains the transistor 525conductive to maintain the enabling voltage on the conductor 530 for atime interval determined by the RC time constant of the switch circuit520, that is, the resistors 522 and 523 and the capacitor 528. Byselecting the value of those parts, the time period that the enablingsignal remains on the conductor 530 may be controlled. If desired, theswitch 531 may be closed to maintain the transistors 521 and 525conductive which causes the enabling signal to be present on theconductor 530 as long as the switch is closed. The positive voltage onthe base of the transistor 521 when the time switch 520 is closed is, byvirtue of the diode 532, isolated from the latching switch 570 so as toprevent undesired operation thereof.

There is also provided an oscillator circuit S40 including a freerunning oscillator 541. The oscillator 541 includes an NPN transistor542 as a feedback network 543 the components of which are adjusted tocause the free running oscillator 541 to oscillate at an audiofrequency, such as, for example, 1,000 hertz. A speaker 545 is coupledbetween the collector and emitter of the transistor 542 through a DCisolation capacitor 546, and the emitter of the transistor 542 isconnected to ground through a normally closed switch 547.

There is also provided a PNP transistor 550 which functions as an ANDdevice, its base being coupled to the conductor 530 and its emitterbeing coupled to the conductor 516. The collector of the transistor 550is direct current coupled to the base of an NPN transistor 551. theemitter of which is coupled through a resistor 552 to the base of thetransistor 542, and the collector of which is coupled to B+. Inoperation, the transistor 52T will become saturated when the fourcontrol tones have been received, thereby to ground the base of thetransistor 550 through the resistor 522. The series of pulses on theconductor 516 cause the transistor 550 to be conductive for the durationof each pulse and to be nonconductive between successive pulses. Theintermittent conduction of the transistor 550 causes similarintermittent conduction of the transistor 551 which in turnintermittently encrgizes the transistor 542. When energized, thetransistor 542 is able to oscillate at the frequency determined by thefeedback network 543 to form an oscillatory signal which is convertedinto single, spaced bursts of an altering tone by the speaker 545.Between pulses, when the transistor 542 has no base bias, the oscillatorportion 541 does not oscillate and no altering tone is developed. It canbe see, therefore, that the output of the speaker 545 will be a seriesof intermittent tones or beeps.

Of course, the pulses on the conductor 516 are continually developed aslong as the receiver is on and the conductor S30 is grounded through theresistor 522 in accordance with the time constant of the timer switch520. There is provided a switch 547 from the emitter of the transistor542 to ground which interrupts operation ol the oscillator 541. Ifdesired, a manual switch may be provided on the conductor 530, so thatthe user can open the same to disable the audio channel.

summarizing, prior to receiving the series of control tones, the puisercircuit 500 is producing a series of pulses 0n the conductor 516 whichis applied to the receiver circuits 230 intermittently to energize them.lf an RF signal is impressed on the receiver circuits 230 while they areenergized, it will be processed and detected, and, if it contains thefirst control tone to which the decoder 240 is to respond, a firstcontrol signal will be developed on the conductor 473, causing thepuiser circuit 500 to produce a continuous supply voltage for thereceiver circuits 230. This control signal persists as long as thecorrect tones are received in the proper order. The pulses and thesubsequent continual supply voltage are also, of course, coupled to theemitter of the transistor 550 in the oscillator circuit 540. However,without more, no altering tone is emitted by the speaker 545 since oneof the inputs to the AND transistor 550 is not present.

lf the sequence of the four control tones is that to which the decoder240 is to respond, a second control signal `will be developed on theconductor 465, commencing with the fourth tone. The second controlsignal opcrt ates the timer switch 520 to provide on the conductor 530an enabling signal. This enabling signal provides the requisite secondinput for the AND transistor 550 and thereby renders same conductive. Aspreviously explained, the signal on the conductor 516 is coupled to theoscillator 541 to cause same to produce a pulsating signal for thespeaker 54S. It can be seen that, during the last control tone when thesupply voltage on the conductor 516 is continuous, the alerting tonegenerated by the speaker 545 would be continuous. After termination ofthe fourth control tone when the signal on the conductor 516 reverts toa series of pulses again, the output of the speaker 545 becomes a seriesof intermittent alerting tones.

There is also provided a second electronic switch 570, but instead ofbeing of the timing variety, it is a latching switch, that is, itdevelops an enabling signal which will last indefinitely untilinterrupted. The electronic switch 570 includes an NPN transistor S71having its emitter grounded and having its base coupled to groundthrough a resistor 572. The base is also coupled through a diode illill)

573 and a resistor 574 to the conductor 465. A resistor 575 and acapacitor 576 are coupled in parallel between the base of the transistor571 and a switch 576a. There is provided a PNP transistor 577 having itsemitter coupled to the supply voltage and having its base coupledthrough a resistor 578 to the collector of the transistor 571. Thecollector of the transistor 577 is coupled back to the base ot' thetransistor 571 through a resistor 579. In addition, there is a biasingresistor 580 between the emitter and the base of the transistor 577. Theelectronic switch 570 provides an enabling signal on the conductor 581,as will be explained, to operate a lamp control circuit 590.

In operation, both transistors 571 and 577 are nonconductive in theabsence of the second control signal on the conductor 465. lf thereceiver receives a signal containing the sequence of the proper fourtones, the control signal will appear on the [connector] conductor 465starting with the fourth control tone. That control signal is coupledthrough to the transistor 571 to render same conductive which. in turnrenders the transistor 577 conductive, to place a positive voltage outhe collector of the transistor 577. Part of this voltage is fed backthrough the resistor 579 to the base of the transistor 571 in aregenerative fashion to provide [and] nn enabling signal on theconductor 582 equal to the supply voltage, and an enabling signal on theconductor 581 essentially equal to ground reference potential. Theenabling signals on the conductors 581 and 582 will persist, even thoughthe fourth control tone has terminated and no control signal is beingapplied to the electronic switch 570, this being due to the regenerativeswitching action. To unlatch" the electronic switch 570 and remove theenabling signals from the conductors 58| and 582, the switch 576er isclosed momentarily grounding the feedback resistor 579. The positivevoltage on the base of the transistor 571 when the latching switch 570is closed is isolated from the timer switch 520 so as to preventundesired operation thereof, by virtue of the diode 573.

The enabling signal on the conductor 581 is applied to a lamp controlcircuit 590, the lamp control circuit including a PNP transistor 591having its emitter coupled to the conductor 516, and its base coupled tothe conductor 581. The collector is direct current coupled to the baseof an NPN transistor 592, the collector of which is coupled to thesupply voltage and the emitter of which is coupled through a resistor593 to the base of another NPN transistor 594, The emitter ot thetransistor 594 is grounded and the collector is coupled through aresistor 595 to a lamp 600. Without the control signal on the conductor465, no enabling signal appears on the conductor 581 (i.e., it is notgrounded), so that the lamp control circuit 590 is not operative.However, if the proper sequence of four control tones is received, whenthe fourth control tone commences, a control signal will be provided onthe conductor 46S which `will result in an enabling signal on theconductor 581 (i.e., it is grounded) to ground the base of thetransistor 591. The series of pulses on the conductor 516 willintermittently energize the transistor 591 which will, in turn, causeconduction of the transistor 592 so as to provide current flow throughthe collcctor-emitterjunction thereof, through the resistor 593 and intothe baseemitter junction of the transistor 594. This will cause currentto flow through the lamp, the resistor 595 and the collector emitterjunction of the transistor 594. The lamp 600 will be lit for a durationequal to the width of the pulse and will be extinguished between pulses.Thus, the lamp provides a blinking elTect so as more easily to attractthe attention of the user. The light will blink on and off indefinitelysince the series of pulses on the conductor 516 occur indefinitely andsince the enabling signal on the conductor 581 is latched in itsgrounded condition. lf the user wishes to turn ot the lamp, he closesthe switch [576] 576a which removes the enabling signal on the conductor581 as previously set forth. Accor'dngly, the oscillator 540 may beviewed as a rst oscillator for applying a first oscillator signal to thespeaker 54S. The pulse S00, together with the control circuit [59] 590may be viewed as a second oscillator which, in the presence of theenabling signal on the conductor 581, will canse a second oscillatorsignal to be produced for energizing the lamp 600. Also, in thisparticular form, the oscillator circuit 540 may be viewed as autilization circuit for the enabling signal on the conductor S30, andthe lamp control circuit 590 may be viewed as a utilization circuit forthe enabling signal on the conductor 581.

In order to minimize the current drain of the communication receiver 220in its standby condition, the pulse width should be many times shorterin duration than the time between pulses. As set forth previously, inone embodiment the duration between pulses was 25 times as great as thepulse width. On the other hand, when the user is being alerted, it maybe desirable to increase the duty cycle or pulse width with respect tothe interval between pulses so that the alerting tone from the speaker545 persists, and/or the lamp 600 is on, for a greater -f percentage ofthe time.

To this end, there is provided a pulse extender circut 610 having an NPNtransistor 611 with its base coupled by a resistor 612 to the conductor582. The collector of the transistor 611 is coupled to one side of thecapacitor 509 in the pulser circuit 500, and the emitter of thetransistor 611 is coupled through a capacitor 613 to the other side ofthe capacitor 509.

When the last control tone in the series of control tones begins, thereis provided a positive enabling signal on the conductor 582 which causesthe transistor 611 to conduct and thereby place the additionalcapacitance of the capacitator `613 in parallel with the capacitor 509,thereby to increase the on-time of the multivibrator circuit 501. Thisresults in an increased duty cycle, which is refiected on the conductor516 as pulses of increased width and decreased time between successivepulses.

Summarizing, the pulser circuit 500 produces a series of pulses on theconductor 516, which are used intermittently to provide a supply voltagefor the various elements in the receiver circuits 230. In a particularembodiment, the pulse width was 15 milliseconds and the time betweenpulses was 360 milliseconds or a 4 percent duty cycle. This means thatduring 96 percent of the time the cornmunication receiver 220 wasdrawing essentially no current, and that during the other 4 percent ofthe time the receiver was drawing stand-by" current. Accordingly, as wasthe case in the first embodiment described, the useful life of thebattery in the communication receiver 220 may be increased theoreticallyby a factor of 25. However, in the embodiment described, the pulsercircuit 500 may reduce this theoretical increase by about 10 percent.Also, the lamp control circuit 590 and the oscillator circuit 540, whenenergized, draw additional current and contribute perhaps 5 percentadditional battery drain. Finally, the first control tone, if the properfrequency, will cause the continuous supply voltage to be developed fora time sufiicient to examine the second tone. If the second tonereceived is not the proper one, the pulser circuit reverts to producingpulses. This may contribute an additional percent drain. Even with theseadditional losses, the useful battery life can be extended by a factorof 18, in this example, over the life of the same battery in a standardreceiver. With the communication receiver 220, battery drain isminimized so as to conserve battery life, not only during standby, butalso while the alerting tone is generated.

Reference is made to the graph of FIG. l0 wherein the waveform 620represents the signal appearing on the conductor S16 which is the outputof the pulser circuit 500, and consists of a series of pulses 621. Forpurpose of illustration, the duration of each pulse is milliseconds, and360 milliseconds lapses between pulses. Accordingly, the receivercircuits 230 are rendered opera tive for the duration of each pulse 621and are inoperaeil) tive between the pulses 621. If the RF signalimpressed on the antenna 221 includes one or more contol tones, theywill be detected in the receiver circuits 230 and will appear on theconductor 231, if the receiver circuits are supplied with a DC voltage.The Waveform 625 consists of a series of four control tones 626, 627,628 and 629, it being assumed that these control tones, in this order,will activate the decoder 240 to produce control signals on theconductors 465 and 473. The first control tone 626 commences at t1, andfor purpose of illustration it is assumed that it lasts for 400milliseconds. Further, it is assumed that, each of control tones 627,628 and 629 lasts for 25 milliseconds, and there being substantially notime lag between successive ones of the control tones.

At t1 when the control tone commences, the receiver circuits 230 areinoperative since no pulse 621 is applied thereto. The first controltone persists until t3 when the next pulse 621 is generated, at whichtime the receiver circuits 230 become energized to process and detectthe RF signal, including the first control tone 626, which is thencoupled to the decoder 240. If the first control tone 626 is at thefrequency to which the first filter in the decoder 240 is tuned, acontrol signal, represented by the waveform 630 will commence on theconductor 473, at t3, which is a few milliseconds after t2. This controlsignal is applied to the electronic switch 510 in the pulser circuitcausing the same to close and provide a continuous supply voltage, whichis indicated by the numeral 622 on the waveform 620. The continuoussupply voltage is applied to the receiver circuits 230 to permit therest of the first control tone 626 in the RF signal to be processed bythe receiver circuits 230 and to be applied to the decoder 240. When thefirst control tone 626 terminates at t4, the control signal on theconductor 473 will persist beyond the time when the second control toneis to commence, thereby to maintain the receiver circuits 230 operativeto process and detect the RF signal containing the second control tone627, which is then coupled to the decoder 240. lf the second controltone 627 is at the frequency to which the second filter in the decoder240 is tuned, the control signal 630 on the conductor 473 will persistand maintain closed the electronic switch 510 so that it continues toprovide the continuous supply voltage. The continuous supply voltageapplied to the receiver circuits 230 permits the third control tone 628in the RF signal to be processed by the receiver circuits 230 andapplied to the decoder 240. When the second control tone 627 terminates,the control signal on the conductor 473 persists beyond the start of thethird control tone, thereby to maintain the receiver circuits operativeto process and detect the RF signal containing the third control tonewhich is then coupled to the decoder 24|). If the third control tone[627] 628 is at the frequency to which the returned first filter in thedecoder 240 is tuned, the control signal 630 will persist beyond thetime when the fourth control tone is to commence, thereby to maintainthe receiver circuits 230 operative to process and detect the RF signalcontaining the fourth control tone which is then coupled to the decoder240. If the fourth control tone 629 is at the frequency to which theretuned second filter in the decoder 240 is tuned, the control signal onthe conductor 473 will be extended to the termination of the fourthcontrol tone at t6. Accordingly, the continuous supply voltage 622 willsimilarly terminate at t6. It should be noted, that, if after thecontrol signal 630 is developed on the conductor 473, the correct nextcontrol tone is not received, the control signal and the continuoussupply voltage 622 will terminate.

If the proper tones are received in the proper sequence, a secondcontrol signal represented by the waveform 631 will appear on theconductor 465 commencing at t5, during the reception of the last controltone 629r r and will terminate with the completion of the last control

